1. Field of the Invention
The present invention relates to a knocking detecting device for detecting knocking in an internal combustion engine.
2. Description of the Prior Art
There has recently been studied, developed and partially practiced a control system for operating an engine under the optimum conditions by detecting occurrences of knocking to control ignition timings for spark-ignition internal combustion engines or fuel injection timings for compression-ignition internal combustion engines, in order to improve engine performances as for thermal efficiency, power output, mileage or fuel economy, and the like.
Spark-ignition internal combustion engines generally have a greater tendency to knock with faster ignition timing and a less tendency to knock with retarded ignition timing. Best engine output and specific fuel consumption are available with an ignition timing just before the occurrence of knocking when ignition timing is slowly advanced, or an ignition timing when there is slight knocking (known as trace knocking). Maximum engine performance can thus be obtained by detecting occurrences of such slight knocking for automatic control of optimum ignition timing, that is, by retarding ignition timing by a predetermined angle when slight knocking is detected, or by advancing ignition timing by a given angle when there is no knocking for a certain period of time. To perform such automatic control effectively, a knocking detecting device is required for detecting knocking generated in combustion chambers of an internal combustion engine with a high degree of sensitivity and a high signal-to-noise ratio.
Conventional knocking detecting devices include combustion pressure sensors for detecting vibratory pressures in combustion chambers, vibration sensors for detecting vibrations of engine walls, and a microphone sensor for detecting knocking sounds transmitted through air. It is known that the combustion pressure sensor has a highest signal-to-noise ratio, and the microphone sensor has a lowest signal-to-noise ratio. The combustion pressure sensor, however, needs to be attached to each cylinder for a multicylinder engine, with the result that the overall system is relatively costly. In addition, the combustion pressure sensor requires a complex sensing element for measuring the pressure of a high-temperature gas in the combustion chamber, is less durable, and thus is practically infeasible particularly for automotive engines.
Therefore, the vibration sensor for detecting engine wall vibrations has heretofore been used in practice. This type of knocking detecting device cannot achieve correct control of ignition timing for internal combustion engines since the device has a low signal-to-noise ratio for knocking detection and hence is unable to detect knocking in combustion chambers with precision, for the reasons described below.
In general, knocking is a phenomenon in which the fuel charge in a combustion chamber burns abnormally to produce pressure waves which cause sudden air-column vibrations in the engine cylinder. The pressure vibration causes combustion chamber walls such as a cylinder wall defining the combustion chamber to produce mechanical vibrations, which in turn are transmitted as a knocking sound to various engine walls. The prior knocking detecting device is designed to detect such engine wall vibrations. More specifically, as shown in FIG. 1, the detecting device K is fastened to an engine wall B such as a cylinder block and comprises a housing or casing H and a vibration detecting element S such as a piezoelectric bimorph element disposed in the housing H for detecting vibrations transmitted to the housing H. The vibration detecting element may comprise a piezoelectric element, a magnetostrictive element, or other elements, which are arranged to detect vibrations on the detector casing with high sensitivity.
Internal combustion engines have numerous sources of vibration, such as cams, valves, bearings and others, which will generate mechanical noise vibrations that are also transmitted to engine walls and detected by the knocking sensor. Thus, the knocking sensor fails to detect knocking with a high signal-to-noise ratio. Frequency analysis of vibratory pressures in a combustion chamber of a gasoline engine upon knocking indicates that there are vibration peak components in a high-frequency range of from about 6 KHz to 10 KHz. To detect vibrations in such a frequency range, the knocking sensor has its vibration detecting element S resonant with respect to knocking vibrations for detecting knocking, or includes a band-pass filter for filtering detected signals to detect a knocking signal. Since vibrations in engine walls of automotive engines include many mechanical noises and vibrations having frequency components which are the same as those of knocking vibrations, it has been inherently difficult with such known detection arrangements to provide a high signal-to-noise ratio for detecting knocking in internal combustion engines. As a result, the prior knocking sensor has been unable to correctly control ignition timing and fuel injection timing, that is, unable to effect correct knocking control, thus failing to gain optimum engine performances as for improved thermal efficiency, power output, fuel consumption.